This invention relates to a coil spring having coils of varying diameters, which are particularly useful in vehicle suspension systems.
Vehicle suspension systems typically include a suspension damper, which is connected at one end to the vehicle body and at the other end to the lower control arm of the vehicle. A coil suspension spring circumscribes the damper and acts between an upper spring seat which is secured to the body of the vehicle and a lower spring seat which is secured to the damper. Preferably, the axis of the coil spring acts along the line of the reaction force of the vehicle wheel, which extends between the center of the wheel to the vehicle body. Since the lower end of the damper can not be connected to the center of the wheel, undesirable transverse forces are applied against the damper. Furthermore, because of space constraints in modern vehicles, packaging considerations severely limit the size of the damper and spring. Accordingly, it is desirable that the compressed height of the coil spring be as small as possible, without allowing the coils to engage one another. When the coils of a spring engage one another, the spring ceases to be able to absorb road shocks, and the ride quality is significantly deteriorated.
According to the present invention, a coil spring is provided in which one of the intermediate coils nests within an adjacent coil when the spring is compressed. If the spring has a sufficient number of coils, more than one of the coils may be designed to nest within an adjacent coil. Accordingly, the spring has an overall compressed height (without the intermediate coils contacting one another) that is less than that of a prior art constant diameter spring. Although the spring is useful in vehicle suspension systems, springs that have a reduced compressed height are useful in many other applications.
According to another embodiment of the invention, the axes of adjacent intermediate coils having different diameters are offset from one another. Accordingly, assuming that all of the coils of the spring have the same pitch, the spring will not compress uniformly since the change in diameter of the coils and the offset between the axis of the coils affects the local stiffness of the spring. Accordingly, one side of the spring will be xe2x80x9csofterxe2x80x9d than the other side of the spring. When the spring is compressed, the spring will bow, or curve. As discussed in U.S. Pat. 4,903,985, the ability of a spring to bow or curve as it is compressed has technical advantages, including the fact that a moment is applied to the lower spring seat opposing the transverse forces applied to the damper, thus improving the operation and durability of the damper and providing better ride characteristics. Furthermore, the pitch of the offset coil may be adjusted to compensate for the effects of the offset axis and smaller diameter. Accordingly, the spring will compress geometrically along substantially straight axes, but due to the offset and smaller diameter of some of the coils of the spring, a moment will still be exerted on the lower spring seat. Since stiffness of the spring is also a function of the cross sectional area of the wire from which the spring is wound, tapered wire may be used to provide a smaller cross section on one side of the spring and a larger cross section on the other side of the spring for at least some of the coils.